Lighting device employing a light guide plate and a plurality of light emitting diodes

ABSTRACT

The present invention relates to a lighting device including a light guide plate, and at least one array of light emitting diodes (LEDs), which LEDs are accommodated in holes arranged in the light guide plate. In some embodiments, the device is characterized by an array of lenses arranged such that light emitted by the LEDs passing the lens array is at least partly directed towards areas of the light guide plate free from holes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 12/445,328, which is a national stage applicationunder 35 U.S.C. §371 of International Application No. PCT/IB2007/054166filed on Oct. 12, 2007, which claims priority to European ApplicationNo. 06122321.0, filed on Oct. 16, 2006, and European Application No.07100361.0 filed on Jan. 11, 2007, both incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to lighting device comprising a lightguide plate and at least one array of light emitting diodes

BACKGROUND OF THE INVENTION

Progress in the brightness, lumen efficacy and affordability of solidstate light sources such as light emitting diodes (LEDs) enables newlighting applications that are no longer restricted to niche markets.LEDs offer several advantages over traditional light sources, such aslong lifetime, low operating voltage, instant on, etc. For these andother reasons, LEDs are becoming more and more suited for making lampsfor several applications such as color variable lamps, spotlights, LCDbacklighting, architectural lighting, stage lighting, etc.

For many lighting applications, the light of a single LED is notsufficient, and light of multiple LEDs needs to be combined to form alight source. One solution is to mix light of multiple LEDs in a lightguide, before the light leaves the lighting device. An example of such asolution is disclosed in the document “LED Backlight designs usingLuxeon high flux light source solutions” by Lumileds, Seattle 2004,http://www.lumileds.com/pdfs/Luxeon_light_source_solutions.pdf,incorporated herein by reference. A backlight based on side-emittingLEDs described in this document is schematically illustrated in FIGS. 1a-1 b. With reference to FIGS. 1 a-1 b, the backlight 100 comprises alight guide 102 provided with cylindrical through holes 104 which arearranged in a linear array along an edge 106 of the light guide. In eachthrough hole, there is provided a side-emitting LED 108, whereby lightfrom the LEDs is coupled into the light guide through the sidewall ofthe through holes, as illustrated by exemplary ray traces 110. The edge106 is preferably reflective, to avoid unintended out-coupling of lightvia the edge.

However, in such a solution when a dense array of LEDs 108 is placedclose to the edge 106 it may occur that light from one LED 108 a isdirected via reflection off the edge 106 (exemplary ray trace 112)towards another nearby hole 104 b and gets absorbed or scattered at theLED 108 b inside this hole. Thus, the lumen efficiency of such alighting device is degraded.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at leastalleviate this problem, and to provide a lighting device with improvedlumen efficiency.

This and other objects that will be apparent from the followingdescription are achieved by means of a lighting device, according to theappended claim 1, comprising a light guide plate and at least one arrayof light emitting diodes (LEDs), which LEDs are accommodated in holesarranged in the light guide plate, characterized by an array of lensesarranged such that light emitted by the LEDs passing the lens array isat least partly directed towards areas of the light guide plate freefrom holes.

Because of the lens array, no or little light strikes the nearby holes.Therefore, the LEDs can be placed closer together and losses due toabsorption or scattering at nearby LEDs in the lighting device arediminished. Overall, the luminous efficiency and power of the lightingdevice can be increased.

Preferably, the lens array comprises at least one row of positivelenses, to readily direct the light in the wanted directions. Alsopreferably, the lens pitch is about ½ of the LED pitch, which providesto a feasible design of the lighting device.

In one embodiment, the lens array is placed between one LED array and areflective edge of the light guide plate such that light emitted by theLEDs in the array and reflected by the reflector is directed by the lensarray to pass between the LEDs in the array. Thereby, losses due toabsorption or scattering at adjacent LEDs/holes in the array arediminished, and the luminous efficiency of the lighting device isincreased. To further boost these effects, the LEDs are preferablyaligned with every second junction between two adjacent lenses in thelens array.

In another embodiment, the lens array is placed between a first LEDarray and a second LED array such that light emitted by the LEDs in thefirst array and passing the lens array is directed by the lens array topass between the LEDs in the second array. Thereby, losses due toabsorption or scattering at LEDs/holes in the nearby array arediminished, and the luminous efficiency of the lighting device isincreased. To further boost these effects, the distance between thefirst LED array and the lens array and the distance between the secondLED array and the lens array are preferably equal. Further, the LEDs ofthe first array may be aligned with the optical axes of every secondlens in a row of the lens array while the LEDs of the second array maybe aligned with the optical axes of every other second lens in the rowof the lens array. Alternatively, the LEDs may be aligned with everysecond junction between two adjacent lenses in a row of the lens array.

Preferably, the holes are cylindrical holes, and the LEDs are preferablyside emitting LEDs, for useful in-coupling of light into the light guideplate.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described inmore detail, with reference to the appended drawings showing currentlypreferred embodiments of the invention.

FIG. 1 a is a top view of a backlight according to prior art.

FIG. 1 b is a cross-sectional side view of the backlight of FIG. 1 a.

FIG. 2 is a partial top view of a lighting device according to a firstembodiment of the invention.

FIG. 3 is a partial top view of a lighting device according to a firstvariant of a second embodiment of the invention.

FIG. 4 is a partial top view of a lighting device according to a secondvariant of the second embodiment of the invention.

DETAILED DESCRIPTION

A light emitting diode (LED) based lighting device according to a firstembodiment of the present invention will now be described with referenceto FIG. 2.

The lighting device denoted 10 comprises a light guide plate 12. Thelight guide plate 12 is transparent and can be made of glass or plastics(such as PMMA or PC), for example.

The lighting device 10 further comprises a linear array 14 of LEDs 16arranged along a reflective edge 18 belonging to the light guide plate12. The reflective edge 18 serves to direct any incident light back intothe light guide plate 12, to avoid unintentional out-coupling of lightfrom the light guide plate 12 via the edge. The LEDs 16 are preferablyside-emitting omnidirectional LEDs.

The LEDs 16 are accommodated in cylindrical holes 20 having a circularlateral cross-section, which holes 20 are arranged in the light guideplate 12. ‘Lateral’ is in relation to the plane of the light guideplate. Each hole 20 has a circumferential side facet (cylinder wall) 22through which light from the accommodated LED 16 is to be laterallycoupled into the light guide plate 12. The holes 20 could be throughholes or holes having an opening towards one side of the light guideplate 12 only.

According to the invention, the lighting device 10 further comprises alens array 24. The lens array 24 is placed between the reflective edge18 and the LED array 14, and it comprises one linear row of positivelenses 26. The lenses 26 may be formed separately or as integral partsof the light guide plate 12. Any gap between the lenses 26 and thereflective edge 18 can be filled with air, for example. The lens array24 serves to direct light emitted from the LEDs 16 and reflected off theedge 18 towards areas of the light guide plate 12 free from or with noholes 20 (or LEDs 16, consequently). Namely, the light is directed topass between the LEDs 16. To this end, in a preferred embodiment, thelens pitch P1 is ½ of the LED pitch P2 (P1=½* P2), and with every secondjunction 30 between two adjacent lenses 26 in the linear lens array 24there is aligned an LED 16. The LED pitch P2 is the distance between thecenters of two adjacent LEDs 16, and “aligned with” means here that animaginary line from the junction 30, which line is perpendicular to themain linear direction of the lens array, runs through the center of aLED 16. Further, the focal strength of a lens 26 should satisfy thefollowing relation 1/f=1/D, where f is the focal length and D is thedistance between the LED array 14 and the lens array 24 (the total lensstrength ftot is formed by passing through lens 26 twice;1/ftot=1/f+1/f=2/f and 1/ftot=1/D+1/D=2/D; hence 1/f=1/D). The focallength f is given by f=(n/(n−1))R1, where n is the refractive index ofthe material of the light guide plate 12 and R1 is the radius of eachlens 26. The refractive index for PMMA is typically about 1.49 and therefractive index for PC is typically about 1.56. The various parametersmentioned above should be chosen such that lens radius R1 is larger than½ of the lens pitch P1 (R1>½* P1), otherwise the lens array 24 cannot berealized. Also, to avoid too curved lens surfaces, which can bedifficult to make, the parameters should preferably be chosen such thatR1>½* P1 is by far not violated. In a feasible design, the radius of thecylindrical holes 18 is 3 mm and the LED pitch P2 is 9 mm. The lenspitch P1 is half the LED pitch P2, that is P1=4.5 mm. For example, whenD=15, the lens radius R1 becomes D*(n−1)/n=5 mm, which is larger thanhalf the lens pitch P1, whereby R1>½* P1 is satisfied.

Upon operation of the lighting device 10, a light ray 32 emitted by theLED 16 a, which light ray 32 otherwise would have been reflected off theedge 18 an into the adjacent hole 20 b (as in FIG. 1 a), is focused by acorresponding lens 26 of the lens array 24 on its way towards thereflective edge 18. After reflection off the edge 18, the light ray 32is again focused by the corresponding lens 26 towards the space 28′between the holes 20 a and 20 b and thus misses hole 20 b. On the otherhand, the direction of a light ray 34 emitted by the LED 16 a, whichlight ray 34 otherwise would have been reflected off the edge 18 towardsthe space 28″ between the holes 20 a and 20 c, is not alteredsignificantly by the lens array 24 since the light ray 34 passes closeto the optical axis of the lens 26 (whereas the light ray 32 passes thelens 26 off-axis and is refracted more strongly). Therefore, the lightray 34 is still directed towards the space 28″ between the holes 20 aand 20 c and consequently misses the adjacent LEDs 16. Overall, the lensarray 24 serves to image reflections of the LEDs 16 at spaces 28 betweenthe real LEDs 16 b, whereby losses due to absorption or scattering atadjacent LEDs are diminished, and the luminous efficiency of thelighting device 10 is increased.

A lens array can also advantageously be used in a second embodiment,wherein two LED arrays are arranged parallel to each other, asillustrated in FIGS. 3-4. The function of the lens array is here toavoid that light from one array is absorbed or scattered at LEDs in theother array.

In a first variant (FIG. 3) of the second embodiment, the lightingdevice denoted 10 comprises a light guide plate 12. The light guideplate 12 should be transparent and can be made of glass or plastics(such as PMMA or PC), for example.

The lighting device 10 further comprises two parallel linear arrays 14of LEDs 16. The LEDs 16 are preferably side-emitting omnidirectionalLEDs.

The LEDs 16 are accommodated in cylindrical holes 20 having a circularlateral cross-section, which holes 20 are arranged in the light guideplate 12. Each hole 20 has a circumferential side facet (cylinder wall)22 through which light from the accommodated LED 16 is to be coupledinto the light guide plate 12. The holes 20 could be through holes orholes having an opening towards one side of the light guide plate 12only.

According to the invention, the lighting device 10 further comprises alens array 24 arranged in the light guide plate 12. The lens array 24 isplaced between the two LED arrays 14 in an in-plane arrangement. Thelens array 24 is formed by cutting or otherwise removing a portion 36 ofthe light guide plate 12, which portion 36 has the form of a lineararray of biconcave or double concave lenses. Left is a linear lens array24 with two rows of opposing positive lenses 26. The lens array 24serves to direct light emitted from the LEDs 16 in one array and passingthe lens array 24 towards areas of the light guide plate 12 free from orwith no holes 20 (or LEDs 16, consequently). Namely, the light isdirected to pass between the LEDs 16 of the other array. To this end, ina preferred embodiment, the lens pitch P1 is ½ of the LED pitch P2(P1=½* P2), the distance Da between the LED array 14 a and the lensarray 24 and the distance Db between the LED array 14 b and the lensarray 24 are equal (Da=Db), and the LEDs 16 a of array 14 a are alignedwith the optical axes 38 a of every second lenses 26 of a row while theLEDs 16 b of array 14 b are aligned with the optical axes 38 b of everyother second lenses 26 of the row (thus, the two arrays 14 a and 14 bare displaced with half a LED pitch P2 in relation to each other).Further, the focal strength of two opposing lenses 26 should satisfy thefollowing relation 1/f=1/Da+1/Db, where f is the focal length. The focallength f is given by f=(½)(n/(n−1))R1, where n is the refractive indexof the material of the light guide plate 12 and R1 is the radius of eachlens 26. The refractive index for PMMA is typically about 1.49 and therefractive index for PC is typically about 1.56. The various parametersmentioned above should be chosen such that lens radius R1 is larger than½ of the lens pitch P1 (R1>½* P1), otherwise the lens array 22 cannot berealized. Also, to avoid too curved lens surfaces, which can bedifficult to make, the parameters should preferably be chosen such thatR1>½* P1 is by far not violated. In a feasible design, the radius of thecylindrical holes 18 is 3 mm and the LED pitch P2 is 9 mm. The lenspitch P1 is half the LED pitch P2, that is P1=4.5 mm. For a system withDa=Db=15, the lens radius R1 becomes 5 mm, which is larger than half thelens pitch P1, whereby R1 >½* P1 is satisfied.

Upon operation of the lighting device 10, a light beam 40 emitted by theLED 16 a′, which light beam 40 otherwise at least partly would havestruck at least one hole 20 b in the adjacent LED array 14 b, is focusedby two opposing lenses 26 in the lens array 24 such that the LED 16 a′is imaged at space 28 b′ between two holes 20 b in array 14 b.Consequently, the beam 40 misses the holes 20 b in the LED array 14 b.Similarly, a light beam 42 is focused by lens array 24 imaging the LED16 a′ at space 28 b″, and so on. Overall, the lens array 24 serves toimage the LEDs 16 a of one array 14 a at spaces 28 b between the LEDs 16b of the other array 14 b, and vice versa, whereby losses due toabsorption or scattering at LEDs in the nearby array are diminished, andthe luminous efficiency of the lighting device 10 is increased.

In a second variant (FIG. 4) of the second embodiment, the LEDs 16 arealigned with every second junction 30 between two adjacent lenses 26 ina row of the lens array 22. Here a light beam 44 emitted by the LED 16a′ is focused by two opposing lenses 26 in the lens array 24 such thatthe LED 16 a′ is imaged at space 28 b′ between two holes 20 b in array14 b. Consequently, the beam 44 misses the holes 20 b in the LED array14 b. Similarly, a light beam 46 is focused by lens array 24 imaging theLED 16 a′is at space 28 b″, and so on.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within thescope of the appended claims. For example, in the first embodiment,instead of a straight reflective edge and linear LED array, the LEDscould be arranged in a curved array along a curved reflective edge,given that the radius of the curvature is considerably larger than theLED pitch. In the second embodiment, the lens array could comprise onlyone row of positive lenses. This requires that the lenses are morecurved. In both the first and second embodiments, the LED pitch-lenspitch ratio could be different, for example 1:1. Also, the first andsecond embodiments could be combined in a single device comprisingseveral parallel LED arrays, one of which is placed along a reflectiveedge of the light guide plate.

1. A lighting device, comprising: a light guide plate defining a plurality of holes, at least one array of light emitting diodes (LEDs) at least partially disposed in the holes; and an array of lenses arranged such that light emitted by the LEDs passing the lens array is directed to pass between the LEDs in the array of LEDs, wherein the lens array is placed between one LED array and a reflective edge of the light guide plate such that light emitted by the LEDs in the array and reflected by the reflective edge is directed by the lens array to pass between the LEDs in the array.
 2. The device according to claim 1, wherein the array of lenses comprises at least one row of positive lenses.
 3. The device according to claim 1, wherein, for at least one lens in the array of lens, the lens pitch is about ½ of the LED pitch and is smaller than about ½ of the lens radius.
 4. The device according to claim 1, wherein the LEDs are aligned with every second junction between two adjacent lenses in the lens array.
 5. The device according to claim 1, wherein the holes are substantially cylindrical, and the LEDs are side-emitting omnidirectional LEDs.
 6. A lighting device, comprising: a light guide plate defining a plurality of holes, at least one array of light emitting diodes (LEDs) at least partially disposed in the holes; and an array of lenses arranged such that light emitted by the LEDs passing the lens array is directed to pass between the LEDs in the array of LEDs, wherein, for at least one lens in the array of lens, the lens pitch is about ½ of the LED pitch and is smaller than about ½ of the lens radius
 7. The device according to claim 6, wherein the lens array is placed between one LED array and a reflective edge of the light guide plate such that light emitted by the LEDs in the array and reflected by the reflective edge is directed by the lens array to pass between the LEDs in the array.
 8. The device according to claim 7, wherein the LEDs are aligned with every second junction between two adjacent lenses in the lens array.
 9. The device according to claim 6, wherein the holes are substantially cylindrical,
 10. The device according to claim 6, wherein the LEDs are side-emitting omnidirectional LEDs. 